Motor gasoline formulations are expected to change in order to meet ever restrictive governmental regulations and competition from alternative fuels, such as methanol. One requirement of these reformulated gasolines is that they be substantially reduced in aromatics compounds, such as benzene. Furthermore, it is expected that governmental regulations will also substantially restrict the amount of light hydrocarbons which can be present, thus establishing the requirement that the gasoline be low in emissions.
While the removal of aromatics from gasolines is beneficial from an environmental point of view, their removal represents a substantial debit on motor octane number. This leaves the refiner in a position of finding a suitable substitute for aromatics from an octane number point of view, but which also meets the low emissions requirement.
One class of compounds which have been proposed for reformulated gasolines are oxygenates, such as the unsymmetrical dialkyl ethers, particularly methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME). If such compounds are to be extensively used in reformulated gasolines, then new and improved methods of their manufacture will be needed.
Conventional methods of manufacture of such ethers are typically based on liquid-phase reactions, such as the reaction of iso-butylene with methanol over cation-exchanged resins (e.g. Amberlyst 15. See Hydrocarbon Processing, Oct. 1984, p. 63). Metal exchanged conventional clays have been used to make ethers, such as the reaction of 1-alkenes to bis-sec-alkyl ethers (Adams, et al., J. Catalysis, 58, p. 252 (1979)); the reaction of ethanol with hex-1-ene (Ballantine et al., J. Mol. Catalysis, 26, p. 37 (1984)). However, in a comparison of ether formation from pentanol over exchanged and pillared clays, the pillared clays were much less active than the non-pillared clay catalysts (Diddams, et al., J. Chem. Soc. Chem. Commun., p. 1340 (1984)). The projected shortage of isobutylene, and other C.sub.4 and C.sub.5 unsaturates as raw materials, provides an incentive for finding alternative reactions for producing such ethers.
One such alternative method is taught in U.S. Pat. No. 4,503,263 wherein an olefin and water in the gas phase, are reacted over a solid heterogeneous C.sub.10 to C.sub.18 perfluorinated alkanesulfonic acid superacid in the olefin and the water in the gas phase, but below about 120.degree. C.
Other alternative approaches can be found in U.S. Pat. Nos. 4,822,921 and 4,827,048 wherein tertiary butyl alcohol is reacted with methanol. In accordance with the '921 patent, these alcohols are reacted in the presence of a catalyst comprised of an inert support, such as titania, having a phosphoric acid impregnated thereon. The catalyst of the '048 patent is a heteropolyacid, such as 12-tungstophosphoric acid or 12-molybdophosphoric acid, on an inert support, such as titania.
Also, copending application, having an Attorney Docket No. of OP-3637, entitled "PROCESS FOR THE PRODUCTION OF UNSYMMETRICAL TERT-DIALKYL ETHERS", discloses a process for preparing the unsymmetrical tert-dialkyl ethers by use of a catalyst comprised of boron trifluoride hydrates on porous inorganic supports.
While such alternative processes for producing ethers, such as MTBE, show promise and may have some commercial value, there is still a need in the art for other alternative processes for producing this potentially important class of ethers.